In the prior art, there is an optical semiconductor element package on which a light emitting element or a light receiving element, and the like is mounted. In the optical semiconductor element package in the prior art, leads that are sealed in a circular header with glass are provided. Also, a light emitting element and a light receiving element for monitoring lights from the light emitting element are mounted on the header, then a cap in which a transparent glass window is provided in its center part is mounted on the header, and then the light emitting element and the light receiving element are hermetically sealed.
In Japanese Laid-open Patent Publication No. 2010-186965, Japanese Laid-open Patent Publication No. 2003-163382, and Japanese National Publication of International Patent Application No. 2005-516404, the related arts are disclosed.
As explained in the column of the preliminary matter described later, in the optical semiconductor element package in which the leads are sealed in the circular header with the glass, even though there is a request that the leads should be placed closer to the surface emitting laser in order to reduce a transmission loss of the transmission path, it cannot easily respond to such request.